Microphones



Jan. 22, 1957 G. R. PoNTzx-:N ETAL 2,778,882

MICROPHONES Filed Nov. 2, 1951 /VEN for( ef/vr United States PatentMICROPHONES Application November 2, 1951, Serial No. 254,567

Claims priority, application Great Britain January 11, 1951 3 Claims.(Cl. 179-1155) This invention relates to microphones.

One of the objects of the present invention is to provide a microphonewhich is largely, if not wholly, non-responsive to background noises,that is to say, noises emanating from a relatively distant source, butwhich is suitable for transmitting sounds applied to the microphone froma close source.

Microphones fullling this object can be advantageously employed inaircraft, in vehicles with a high ambient noise level, in crowdedsurroundings and under noisy conditions in general.

According to the present invention there is provided a microphonecomprising a plane diaphragm, both faces of the diaphragm beingsubstantially freely accessible to the surrounding air oversubstantially the whole peripheral extent of the diaphragm, means forsupporting the diaphragm so that the latter can vibrate, a coilsupported from the diaphragm to move in correspondence with thevibration of the diaphragm, and means for establishing a magnetic iieldwhereby movement of the coil in the ield causes an electromotive forceto be induced in the wires of the coil, the arrangement being such thatsound waves from a relatively distant source cause substantially novibration of the diaphragm and sound waves from a relatively closesource not in the plane of the diaphragm cause vibration of thediaphragm.

We are aware that it has been previously proposed in United StatesPatent No. 2,350,010 to provide a microphone having a peripherallysupported diaphragm mounted within a housing such that the diaphragmdivides the housing into two compartments, there being two openings inthe housing, one leading into each compartment so that if vibrations orsound waves are applied equally at the 50 housing openings, the waveswill act equally upon the diaphragm to prevent vibration thereof. When,however, sound waves are emitted in close spaced relation to, and aredirected into, one ofthe openings of the previously proposed microphone,the diaphragm vibrates, whereby a 5r coil is moved correspondingly in amagnetic eld and an electromotive force is generated in the wires of thecoil.

in the previously proposed microphone, the housing and openingsconstitute means for directing sound waves initiated at a distance toimpress these waves upon the two opposite faces of the diaphragm insubstantially identical phase and amplitude. In the microphone accordingto the present invention there is no such means for directing the soundwaves, the diaphragm being substantially freely accessible to thesurrounding air over substantially the whole peripheral extent of thediaphragm.

For a better understanding of the invention and to show how the same maybe carried into effect, reference will now be made to the accompanyingdrawings, in which:

Figure 1 is a cross-sectional elevation of a microphone having acentrally supported diaphragm;

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Figure 2 is a plan view of the magnet housing of the microphone shown inFigure 1, and

Figure 3 is a cross-sectional elevation of a microphone having aperipherally supported diaphragm.

The microphone shown in Figure 1 has a pot-type magnet comprising apermanent magnet 1 having a centrally disposed pole-piece 2 securedthereon. A magnet housing 3 of hollow cylindrical form closed at thelower end by a base plate 3A encloses the magnet 1 and pole-piece 2. Thepole-piece 2 is disposed within a circular opening in the top of thehousing 3, whereby an annular air gap 4 is formed between the pole-piece2 and the housing 3. The iron circuit of the magnet consists ofpole-piece 2, magnet 1, base plate 3A and housing 3, the polepiece 2being of opposite polarity to that portion of the housing adjacent thepole-piece.

A coil 5 disposed in the air gap 4, is wound on a former 6, which latteris supported from a diaphragm 7. The diaphragm 7 is secured on thepole-piece 2 by means of an upstanding stud 8 which is passed throughthe centre of the diaphragm 7 and has a nut 9 threaded thereon, A thickwasher 10 is interposed between the diaphragm 7 and the pole-piece 2,and a washer 11 is interposed between the nut 9 and diaphragm 7.

A plurality of recesses 12 (Figure 2) are formed around the upper outerperiphery of the housing 3 to ensure that the underside face of theperipheral edge portion of the diaphragm is freely in contact with thesurrounding air. The 'recessed top of the housing 3 has a castellatedappearance.

The diaphragm is protected against damage by a perforated cover 13, thebottom of which is secured to the side wall of the housing 3. lt is tobe understood that the cover 13 allows sound waves to be transmittedfreely therethrough and in no way serves as a guide for directing soundwaves onto the two opposite faces of the diaphragm 7.

A washer 15 is provided within the housing 3 for closing the cavitybetween the housing 3 and the magnet 1.

The diaphragm 7 has a plurality of holes 16A therethrough, the holesbeing disposed in a ring around the washer LD.

The microphone shown in Figure 3 is similar to Figure l except that thecentral support for the diaphragm is omitted and instead the diaphragmis supported at its periphery. For this purpose, a plurality of pillars14 screwed into the housing 3 support a mounting ring 17. A diaphragm 16is secured between the ring 17 and a clamping ring 1S which latter issecured to the pillars 14. The diaphragm 16 has an aperture 19 at thecentre thereof.

The housing 3 shown in Figure 3 tapers towards the diaphragm 16 so thatthe underside face of the diaphragm 16 has free access to thesurrounding air.

The microphones described above with reference to the gures, operate asfollows. Sounds originating from a relatively distant source causepressure waves to impinge simultaneously and substantially equally onthe upper and under-sides of the diaphragm. The forces acting on theupper and under-sides of the diaphragm are, therefore, substantiallybalanced and substantially no vibration ot the diaphragm takes place.Hence substantially no electrornotive force is generated in the coil 5.When sound waves emanate from a relatively close point source (forexample, a persons mouth) above the microphone as seen in Figure l or 3,the pressure waves impinge upon the upper side of the diaphragm withgreater intensity than they do on the under-side of the diaphragm. Hencethe forces acting on the diaphragm are unequal and the latter vibrates.The coil S moves with the diaphragm 7 or 16 as the case may be, and cutsmagnetic lines of force across the air gap 4, whereby an electromotiveforce is induced in the coil.

We claim:

1. A microphone comprising an effectively plane diaphragm, supportingmeans for the diaphragm permitting vibration of the latter, said meansengaging the diaphragm over an area which leaves substantially the Wholeof the peripheral extent of the diaphragm substantially freelyaccessible to the atmosphere surrounding the microphone whereby bothfaces of the diaphragm become substantially freely accessible to theatmosphere surrounding the microphone, a coil supported from thediaphragm to move in correspondence with the vibration of the diaphragm, means for establishing a magnetic eld whereby movement of thecoil in the eld causes an electromotive force to be induced in the wiresof the coil, and the diaphragm having at least one aperture within thearea covered by the coil, the arrangement thus being such that soundwaves from a relatively distant source cause substantially no vibrationof the diaphragm and sound waves from a relatively close source not inthe plane of the diaphragm cause vibration of the diaphragm.

2. A microphone comprising an effectively plane diaphragm, both faces ofthe diaphragm being substantially freely accessible to the airsurrounding the microphone over substantially the whole peripheralextent of the diaphragm, a coil of annular form supported from thediaphragm to move in correspondence with the vibration of the diaphragm,a pot-type permanent magnet having an annular air gap in which the coilis located, whereby movement of the coil causes an electromotive forceto be induced in the wires of the coil, a support rigidly securing thecentral portion of the diaphragm to the central pole of the pot-typemagnet, and the diaphragm having a plurality of holes arranged in a ringaround said support.

3. A microphone comprising an eectively plane diaphragm, both faces ofthe diaphragm being substantially freely accessible to the airsurrounding the microphone over substantially the Whole peripheralextent of the diaphragm, a coil of annular form supported from thediaphragm to move in correspondence with the vibration of the diaphragm,a pot-type permanent magnet of cylindrical form and having an annularair gap in which the coil is located, whereby movement of the coilcauses an electromotive force to be induced in the wires of the coil, asupport rigidly securing the central portion of the diaphragm to thecentral pole of the pot-type magnet, and there being recesses at theouter periphery of the magnet adjacent the diaphragm to provide freeaccess of the surrounding air to the face of the diaphragm adjacent themagnet.

rReferences Cited in the le of this patent UNITED STATES PATENTS2,002,189 Round et al. May 21, 1935 2,238,741 Lauer Apr. 15, 19412,293,078 Proctor Aug. 18, 1942 2,534,157 Zuerker Dec. 12, 19502,537,253 Andersen Ian. 9, 1951 2,549,963 De Boer Apr. 24, 1951 FOREIGNPATENTS 556,412 France Apr. 12, 1923 505,407 Germany Aug. 20, 1930599,424 Germany July 2, 1934 679,439 Germany Aug. 5, 1939 936,121 FranceJuly 9, 1948

